The present invention is generally related to electronic protective devices and more specifically is related to a direct current control circuit for reducing built-in relay switching hysterisis in the overload protection circuit of a thermoelectric generator.
The prior art direct current overload control apparatus suggested for use in connection with thermoelectric generators have employed solid-state switches and isolated relays. However, a solid-state switch presents a serious power loss, an output power reduction of 7-15% and, in addition, is not reversible. The isolated relays, due to their built-in hysterisis, require a relatively large input voltage reduction for resetting, which may prevent the generator from providing sufficient voltage to operate ancillary items.
In a thermoelectric generator direct conversion of heat energy into electrical power is obtained using a suitable semi-conductive material having a relatively low electrical resistance and good heat insulation. A semi-conductive material such as bismuth telluride is selected, which operates below 300.degree. F. One portion of the material is heated and the other portion is kept relatively cool, creating a "Seebeck voltage" between the two portions. That voltage difference may be 10 millivolts for a 100.degree. F. temperature difference. For higher temperatures, about 1000.degree. F., one may use as the semiconductor material a lead telluride material. A series of semi-conductive thermocouples, for example, of NPN-PNP, is formed to obtain higher voltages in the range up to 100 volts or even higher. The heat may be obtained by burning liquid hydrocarbon fuels such as diesel fuel and gasoline or by burning other conventional fuels such as propane.
This type of generator is particularly important when dependability and noise are problems. The thermoelectric generator, compared to a gasoline driven piston engine and generator set, is relatively silent and dependable, maintenance-free, low in cost and may be used with a variety of fuels. For example, such thermoelectric generators may be a liquid fueled thermoelectric generator (TEG) used by the military to provide power in the 0.1 to 2.0 kW range with high reliability and total silence. The electric power obtained from the TEG generator may be sufficient to power communication equipment or to charge batteries.
The thermoelectric generator is itself a user of the electrical power it produces. Of course, the generator must be started by outside power, for example, using a battery; but, once started, it supplies its own electrical power to maintain its continued operation. The thermoelectric generator may use electrical power for "ancillary items" which constitute a "secondary load" and include such components as the electric fuel pump for the liquid fuel, the igniter to re-start the burner, the atomizer for the fuel, the cooling fan to help cool the relatively colder portions of the generator, and the blower fan to maintain proper oxidation of the fuel.